Getting 'Kinky': Nanoscale Atomic Interfaces Have Serious Flaws

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Scientists have long believed that the strength of a material
lies in the way it is structured at the molecular level.

For example, shatter points and grain boundaries, found in
substances like crystals and metals, are structural flaws in the
way the compound's individual molecules or crystals hold
themselves together. These flaws often become the site of
fracturing, corrosion and weakness.

Using
nanoscopic tools, scientists have found a way to correct
these types of defects by changing the way the individual
molecules or crystals interface with each other to form a
structure.

The "perfect" means of structuring a crystal or metal, called a
"coherent twin boundary," was discovered in 2004. Coherent twin
boundaries were considered "perfect" because they appeared to be
thin, perfectly flat planes of atoms.

Forming these coherent twin boundaries at the nanoscale in
materials like gold and copper makes them not only stronger, but
also more malleable and durable. The metals were also shown to
become more conductive as well, greatly increasing their
effectiveness in electronic hardware applications.

Using a high-resolution electronic microscope, researchers at the
University of Vermont researchers examined
coherent twin boundaries in a sample of copper and saw, to
their surprise, that the bonds weren't straight at all — in fact,
the bonds had small kinks and curves that made them look more
like rickety stairs than straight lines.

"We had no idea such defects existed. So much for the perfect
twin boundary. We now call them defective twin boundaries,"
Frederic Sanzos, a University of Vermont engineer, said in a news
release.

With their jagged lines and arbitrary angles, the coherent twin
boundaries in copper may not be as pretty to look at anymore, but
that doesn't mean they don't work. In fact, the researchers found
that the strength of the coherent twin boundary is actually
because of these "flaws," not in spite of them.

The discovery that the coherent twin boundaries in copper are
"inherently defective" means that researchers will have to go
back and re-examine coherent twin boundaries in other materials
as well. It also opens up the question of why the so-called
defects actually contribute to the material's strength.

"There are all manner of defects in nature… The point of this
paper is that some defects make a material stronger," Sansoz
said.